The diploma thesis is focused on the thermal balance of the pressure autoclave type AZ 240 used for the color modification of the oak tree with saturated steam. During the technological processing of the wood by steam, the wood gets favorable but also unfavorable changes in its properties, and it is reflected in the use of this wood.In the beginning of the thesis there is an analysis of the processing of the oak wood by steam, there are also chemical reactions which are responsible for the color modification of the wood. The technological equipment used for processing of the wood by pressurized steam includes bells, which are currently less used, and pressure autoclaves.The thermal balance of autoclave is analyzed by using a mathematical model designed for autoclave. The result of the balances of the individual regimes used in the autoclave is that highest heat consumption is used for heating wood, about 73, 5 % of the total heat delivered to the autoclave during the technological process of the thermal modification, which indicates the high efficiency of the autoclave.By setting the standard of the technically justifiable pressure autoclave AZ 240 has been shown that by increasing the temperature in the autoclave and the prolonged time of the action of the saturated water vapor on wood results in higher heat consumption which is also reflected in the final QTZN standard.